A number of gas discharge display panels in the prior art are generally characterized as being segmented electrode display systems and these typically incorporate a plurality of groups of cathode segments, each group being arranged in a character display position, typically in a figure "8" configuration, and a dielectric layer having openings therein at the segments for masking the interconnecting printed circuitry from the gaseous medium. An anode electrode, typically a transparent tin oxide coating on a cover plate or perforated element grid is provided, there being one anode for each group of characters. In addition, there is typically provided a keep alive discharge which provides a supply of free electrons and ions for priming and conditioning of the panel so as to avoid or minimize statistical ionization time delays inherent in gas discharge displays from the application of the voltage to a particular anode-cathode grouping to the onset of the discharge.
Known voltage supply systems include voltage stabilization means for relating the voltage applied to the anode-cathode electrodes directly to some function or condition taking place inside the panel so that the circuitry self adapts voltage supplied to the anode-cathode electrodes to various tolerances encountered in the manufacturing of such display panels and to changes occurring after a period of use.
In one prior art system invented by the inventor hereof a single high voltage transistor had its base electrode connected to a reference electrode. Such high voltage transistors normally have low current gain and on heavy current loads, such as when the number 8 is being displayed, the system begins to draw current from the reference electrode and the reference then goes out. This defect is avoided in accordance with the present invention by providing a PNP-NPN transistor couple to achieve a very high gain voltage follower with very small loss and isolates or provides a block of variation in loading from affecting the reference. This permits the flow of the minimum current to the reference electrode and still sustain a very high signal current to the load, the anodes in the display. The emitter of the NPN transistor and the collector of the PNP are commonly connected to a large resistor and the anode-cathodes of the display, constituting the load, are connected to this point, each anode being isolated by a blocking diode. The transistor couple functions as a voltage follower with high current gain and the isolation of the source of control voltage from the load is effective over a wide range of load current.
In a typical arrangement, the keep alive electrodes of a gas discharge display panel have applied thereto a fixed direct current voltage which initiates a discharge at the keep alive anode-cathode electrode pair and, in conjunction with a resistor between one of the electrodes and the electrode pair, provides a measuring circuit for measuring one or more conditions existent in the tube. This measured voltage then is applied to a control circuit which includes data sources such as MOS chip circuits. Normally the voltage comes up slowly because it is from a filtered supply. However, in some cases, the power supply is arranged so that when the direct voltage is applied to the keep alive electrodes the turn on switch is not on the alternating current side of the supply but rather is on the direct current side of the high voltage supply so that there is always a high voltage which is applied through a switch to the circuit. This establishes a sharp step function which is coupled directly to the MOS chip and in the case of a capacitor drive circuit, there is a rate of rise of voltage due to the charging current for the capacitor which develops enough voltage to possibly destroy the MOS chip.
In accordance with a further feature of the present invention, the rate of rise of this voltage is controlled by providing a coupled pair of voltage gain transistors with a capacitor connected between the collector and emitter of the input transistor so that the rate of rise of the voltage at the output of the voltage follower is a ramp voltage. Thus, the control can be stepped to any rate desired and if the rate of rise of voltage is controlled, then the maximum voltage drop in the system is easily determinable and hence controlled.